The present invention resides in an apparatus for forming a threaded, molded plastic article, as plastic closures, and particularly for removing said article from the mold.
The art of injection molding plastic closures is well known in the industry. Dependant upon the design of the plastic closure, including the thread type and number of complete threads, it may be ejected from the mold in a variety of ways. If the plastic material being molded is flexible or resilient it may be stripped by a commonly known stripper ring pushing the article off the core. The molded part must be solidified enough not to be folded over onto itself during ejection but sufficiently elastic to return to essentially its original molded shape after the threads have been stretched over the core. If the molded material does not possess the appropriate characteristics of flexibility for this most economical and simple method of ejection, the article will be damaged or may not return to its original shape and size. Similarly, a very defined or deep thread profile may be inherently prone to stripping damage. Also, the part may have other delicate or fragile features such as a commonly known tamper evident ring, which could be damaged even if an otherwise acceptable plastic is being injection molded.
For such situations where the molded part will be too rigid or fragile to simply push off the core, a collapsible core could be used such as the type shown in U.S. Pat. No. 4,130,264. This patent shows a plurality of thread forming segments situated around the periphery of the core which move on tracks to collapse the core inwardly, so that the molded article can easily be pushed off.
In the case of molding a cap having an integral tamper evident ring, an alternative method is shown in U.S. Pat. No. 4,155,698. Here the first female portion of the cavity which surrounds the threaded portion of the closure is removed from the molded closure, while a second female portion adjacent the tamper evident ring stays in place. The core is then retracted while the closure stretches elastically over it. This is possible because the closure device is held fast by the second female portion in the region of the tamper evident ring. The part can then be ejected by a core pin pushing on the inside of the closure or cap, as the inward elastic yielding of the fragile tamper evident ring is permitted.
When molding a plastic resin which is too rigid to be stripped from the threaded portion of the cavity, without permanent deformation of the molded article, one may use a method as shown in U.S. Pat. No. 4,652,227. In this patent, an unscrewing chuck is used to come over the molded part once the female portion of the cavity is removed. The chuck grasps the outer feature of the closure, then rotates the closure as it moves slidably backward to facilitate the axial movement of the unscrewing closure. The patent also teaches that the unscrewing function can be accomplished, separately and simultaneously, outside of the molding area so as not to delay the injection molding of more closures. This method has the advantage of a simpler mold design which requires no moving parts for ejection of the article; however, the unscrewing system can become quite complicated and costly as it is virtually a machine unto itself.
Another method of unscrewing is accomplished by first separation of the mold halves (opening the mold), then rotating the core portion of the mold cavity while a stripper ring or bottom forming portion of the cavity moves slidably forward in timed relationship with the axial movement of the closure, until the closure is clear of the core portion and allowed to fall free from the molding area. The stripper ring will often employ protruding features, such as teeth or notches, which will keep the molded article from turning in the direction of the rotating core. The stripper ring may be moved forward by a sliding cam attachment that is linked to the rotation drive of the core, thereby ejecting the closure from the core.
This approach carries with it various disadvantages and short-comings. If the core rotation and stripper ring plate movement are directly linked to a motive means which in turn is not directly linked to the mold closing means, there is the likelihood of collision damage should the mold closing means complete its task before the stripper ring return means. To eliminate this possibility, the stripper ring return means must be completed in sequence before the mold closing begins. These non-simultaneous movements can undesirably add one or two seconds to the entire mold cycle time.
A further disadvantage of the system is found in the molded article cooling time required.
For example, if water cooling is desired in the core portion to hasten solidification of the injected plastic, it is necessary to employ rotary water seals around the core to prevent water leaks. These seals will wear out with time due to rotational friction and must be replaced before the imminent leak occurs. In addition, the cores typically have a small drive gear (to maintain overall compactness) which restricts the water channel size in the core. A further disadvantage is evidenced in that the core portion is long and slender and as such, it is prone to wobble as it rotates, causing premature wear on the mating stripper ring surface. This causes a gap between the normally shut-off surfaces between the stripper ring and the core and allows the plastic melt to squeeze into the space (commonly known as "flashing" of the part). Costly repairs and down-time are the result of this occurrence. Of course, gear wear is also to be expected as the rack is used to drive the core rotation for the ejecting of the article as well as for the reregistering of the rack into its original position, ready for the next ejection stroke. Still a further disadvantage includes the excessive overall length of the core assembly, which incurs higher material costs and manufacturing difficulty, as well as limiting the number of available injection molding machines with sufficient clamp stroke to accept the overall mold shutheight. This has been found to be especially true for multi-level or stack molds.
It is, therefore, the principal object of the present invention to provide an improved apparatus for forming a threaded, molded article, as a plastic closure and particularly for removing said article from the mold.
It is a further object of the present invention to provide an improved apparatus as aforesaid which is compact and durable and is capable of more rapid production without undue wear.
It is a still further object of the present invention to provide an apparatus as aforesaid which permits a mold core cooling channel of maximum size.
Further objects and advantages of the present invention will appear hereinbelow.